Publications referred to throughout this application are hereby incorporated by reference into this application in their entireties to more fully describe the state of the art to which this invention pertains.
With the popularity of refractive surgery rising, calculating intraocular lens (IOL) power after refractive surgery is becoming increasingly important. The calculation is often incorrect because of the difficulty in obtaining an accurate central corneal power in a post-refractive surgery patient. The measured keratometric values for previously myopic patients are usually higher than the actual power, leading to hyperopic results. Conversely, the measured keratometric values for previously hyperopic patients are usually lower than the actual power, leading to a myopic result. Both inaccuracies lead to an inability to meet our patient's rising expectations.
Intraocular lens power calculations in eyes with previous refractive surgery remains difficult largely due to the inaccuracy of traditional keratometry measurements. Keratometry readings which are steeper than actual values which occur in previously myopic eyes lead to underestimation of IOL power and a hyperopic result. Keratometry readings which are flatter than actual values which occur in previously hyperopic eyes lead to overestimation of IOL power and a myopic result. Traditional keratometers do not account for the asphericity of the cornea and after refractive surgery the asphericity of the cornea decreases. The anterior curvature changes while the posterior surface is not altered. Changes in this relationship account for errors in traditional keratometry readings and this error becomes increasingly significant with higher corrections (8, 13, 15, 22).
Our study supports both of these findings. For myopic eyes, both IOLflatK and IOLavgK are based on post-LASIK keratometry readings, and both result in under correction (−1.13 and −0.92 respectively). Also, each of these methods was further deviant for higher amounts of pre-LASIK myopia. These deviations were statistically significant (p=0.0036 and p=0.0094, respectively). Conversely, for hyperopic eyes IOLavgK and IOLsteepK resulted in overcorrection (1.36 and 0.75). Furthermore these 2 methods showed a tendency to further overcorrect for increasing amounts of pre-refractive surgery hyperopia, however this was not statistically significant.
Currently the most widely used method for keratometric calculation post refractive surgery is the clinical history method. Table 1 below shows a review of the current case series and reports in the literature. Most of these series used the clinical history method and found it the most accurate of the different methods of determining central corneal power. Accurately calculating this method can pose some challenges.
TABLE 1Review of Case Series/ReportsSummaries of case series of cataract surgeries following excimer refractive surgeryYearCasesMethod of calculationFindings/SuggestionsAuthorArgento et al.920037MultipleClinical history method most reliableOdenthal et al (12)200215MultipleClinical history method most reliable,combined with Hoffer QRandleman et al. (16)200210MultipleClinical history method, contact lensmethod, and average of 2Ladas et al. (17)20012TopographyHyperopic result, needing piggybacklensGimbel et al. (19)20016Clinical History, ManualClinical history method more reliablethan manual keratometryGimbel et al. (20)20005Clinical History, ManualClinical History method or use offlattest K most reliableKalski et al (21)19974MultipleClinical history method most reliableCase ReportsLesher, et al. (23)19941Manual keratometryHyperopic Result (+1.60)Morris, et al. (24)19981Manual keratometryHyperopic Result (+4.50)
The pre-cataract extraction SEQ is a value needed to determine the total change in SEQ caused by refractive surgery. Error can be encountered when myopia induced by nuclear sclerosis is incorporated into this value, leading to inaccurate change in SEQ and error in IOL power calculation. For example, in our study, after refractive surgery the difference in mean SEQ prior to cataract formation and after cataract formation was −1.09. Therefore, it is important to account for myopia induced by nuclear sclerosis. Variations in clinical history method include keeping the SE at the corneal versus the spectacle plane. Studies have shown that using the spectacle plane refractions in the clinical history method are more accurate (11, 12, 17). Our study also shows this for myopic patients, as the mean deviation of IOLHisK from IOLexact was −1.76D whereas the mean deviation of IOLHisKs from IOL exact was only −0.56D. For hyperopic patients the difference between IOLHisK and IOLHisKs from IOLexact was similar (0.70D and 0.84D, respectively). Finally, this method assumes that there is a one to one relationship between change in refraction and change in central corneal power, which may not hold for higher refractive corrections (13). Our study shows that there is a tendency for IOLHisK and IOLHisKs to be more inaccurate with increasing amounts of pre-refractive surgery myopia and hyperopia (FIGS. 2 and 3). However these deviations are not statistically significant.
Alternative methods for corneal power calculations include the contact lens method. This is determined by the difference in manifest refraction before and after insertion of a plano hard contact lens with a known base curve. The change in refraction is subtracted from the known base curve and this is assumed to be the curvature of the central cornea (2). This method was evaluated for patients with normal corneas with and without cataracts and was found to be accurate in patients with vision as low as 20/80 due to cataracts (3). The corneal power calculation becomes unreliable when visual acuity is less than 20/80. Other issues involved in the contact lens method in post refractive surgery patients include adequate hard contact lens fit.
Both of these above methods rely on either pre-refractive surgery measurements and/or pre-cataract surgery measurements. If these are unavailable the current options for measuring central corneal power include conventional keratometry (CK) (either automated or manual) or corneal topography (CT). Studies have shown that measurements obtained on post surgical patients with corneal topography are more accurate than conventional keratometry (4-7). This result is likely due to the fact that corneal topography measures over 1000 points in the central 3.0 mm zone, while conventional keratometry only measure 2 points located 3.2 mm and 2.6 mm from the corneal center.
Unfortunately, both CK and CT calculate the average corneal power using an effective index of refraction of 1.3375. This value assumes an almost spherical relationship of the cornea and a constant difference between the anterior and posterior curvature. In refractive surgery this value is not reliable as ablation changes curvature principally on the anterior surface and the posterior surface is changed to a lesser extent. Measurement of the posterior corneal curvature remains difficult. Also, how the posterior curvature changes after refractive surgery remains controversial. One study by Seitz, et al showed a mean increase in posterior curvature of 0.11D in 57 eyes following myopic LASIK. Furthermore, the higher the attempted correction, the more of an increase in posterior curvature (10).
Newer methods of measuring corneal topography are becoming available, which might make measuring keratometry post refractive surgery more reliable. These include pan corneal scanning slit-beam topography, stereoscopic topography (14). Once a more accurate measurement of posterior surface curvature is available, it would also be possible to use Gaussian optics formula to better estimate the total power of the central cornea.
The vertex IOL power is another method to estimate IOL calculations in post refractive surgery patients. A study by Feiz et al. examined this method, where the IOL power for emmetropia is based on pre-LASIK keratometry values (13). The spherical equivalent change due to LASIK was then used to modify the IOL power, assuming a diopter of change in IOL produced only 0.7 diopter change in refraction at the spectacle plane. This is based on the IOL position behind the iris and with a vertex distance of 12-13 mm. This method produced higher IOL powers post myopic LASIK and lower IOL powers post hyperopic LASIK. Further, the higher the amount of treatment the more inaccurate traditional keratometry readings would be. Based on their results, this study created a nomogram based on linear regression analysis.
A study by Hamed et al. showed that post refractive surgery keratometry readings (either conventional or topography) could be modified to estimate corneal power. Specifically, using the EyeSys Corneal Analysis system, the authors recommend that the corneal power should be reduced by 15% of the total refractive change induced by surgery. If standard keratometry is used, this value should be reduced by 24%. This study found that standard keratometry was less accurate than the value obtained by the EyeSys (8). Both of these studies present new theories on predicting IOL power post refractive surgery based on manual keratometry and/or topography readings. Actual clinical results have not been reported.
In our study the vertex IOL and a back calculated IOL methods were analyzed. For myopic eyes, both methods interestingly produced an IOL which was stronger on average than IOL exact (1.51D and 1.06D, respectively), unlike the other methods which produced under corrected IOL. Also, these two methods showed a trend toward increased overcorrection from the intended power for higher amounts of pre-refractive surgery myopia; however it did not achieve statistical significance. For the hyperopic group, these two methods calculated an overcorrected IOL as well (0.14D and 0.58D, respectively) and showed a tendency to further overcorrect for higher amount of hyperopia.
Overall, for the myopic group, IOLHisKs produced the most accurate results followed by IOLBC, IOLvertex, IOLflatK, IOLHisK and finally IOLAvgK when compared to IOLexact which would theoretically have given an emmetropic result. Of note there was no statistical difference between IOLHisKs and IOLexact for myopic eyes. For the hyperopic group, IOLvertex was most accurate, followed by IOLBC, IOLHisK, IOLHisKs, IOLSteepK, and finally IOLAvgK. For this group only IOLvertex and IOLBC showed no statistical difference to IOLexact. Based on these results we would recommend using IOLHisKs method for previously myopic eyes, and IOLBC or IOLvertex for previously hyperopic eyes.
When using IOLHisK, IOLHisKs, IOLvertex, and IOLBC it is necessary to obtain pre-refractive surgery data, which includes manifest refraction and keratometry. Unfortunately, this data is not always present. If not present, a practitioner would have to rely on the IOLavgK or IOLflatK (myopic eyes)/IOLsteepK (hyperopic eyes). Our study shows that using these values without an adjustment would produce a large under correction for myopic eyes and a large overcorrection for hyperopic eyes. In our study there was a statistically significant linear regression of deviation of IOLflatK (p=0.0036) and IOLavgK (p=0.0094) to pre-refractive surgery SEQm. Based on this regression a more accurate IOL power can be predicted. For IOLflatK a prediction for adjustment would be: −(0.47x+0.85) where x=pre-refractive surgery SEQm. In this situation only pre-refractive surgery SEQm needs to be known, which is usually easier to obtain from the patient (old pair of glasses) than the entire pre-refractive surgery data. Using paired t-test, the adjusted IOLflatK was found to have no statistical difference to IOLexact, and thus can be used an new formula to predict more accurate IOL powers. An adjustment to IOLavgK was found to be statistically significant as well, and this may prove to be useful once further prospective data can be used.
A Clinical Example would be as Follows:
    Pre-refractive surgery myopic spherical equivalent (SEQm): −6.50 D    Flattest keratometry reading: 42.18 D    Using flattest keratometry reading into SRK/T formula IOL for emmetropia calculated IOLFlatK: 17.76 D    Adjustment based on pre-refractive surgery SEQm:    Adjustment of IOLflatK=−(0.47x+0.85) (x=pre-refractive surgry SEQm)−(0.47(−6.5)+0.85)+2.20    Adjusted IOLflatK=17.76+2.20=19.96 D
Based on this example, the adjusted IOL would provide a more accurate result post-operatively.
For hyperopic eyes, linear regression was performed on all methods compared to pre-refractive surgery hyperopia. The only statistically significant relationship was IOLvertex (P=0.021). This data, however, requires pre-operative keratometry and manifest refraction is still needed for this method.
A clinical example of an adjustment to IOLvertex based on pre-refractive surgery SEQh is as follows:    Pre-refractive surgery SEQh: +2.00    IOLvertex: 20.00    Adjustment of IOLvertex: −(−0.80x+2.69) where x=pre-fractive surgery SEQh −(−0.80(2.00)+2.69−1.09    IOLvertex +Adjustment: 20.00 D−1.09 D 18.91D